The objective of this project is description of mechanisms of bacterial evasion and/or subversion of the killing mechanisms of phagocytic cells by intracellular and cell-associated parasites. Both Chlamydia psittaci and Coxiella burnetii replicate within guinea pig peritoneal macrophages (PEC), thus, are able to avoid the antimicrobial mechanisms of these phagocytic cells. Neither induces macrophage oxidative killing mechanisms when assessed by stimulation of chemiluminescence or hexose monophosphate shunt activity. Mycoplasma pulmonis, a surface-associated parasite, induced in rat PEC a chemiluminescent burst equal in peak height to that induced by either Pseudomonas aeruginosa or Staphylococcus aureus. In contrast, mouse or guinea pig PEC not only failed to respond with an oxidative burst to M. pulmonis, but were inhibited in their response to subsequent challenge with S. aureus. This inhibition was both multiplicity and energy dependent.